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Structure of prominence legs: Plasma and magnetic field  

Peter James Levens   Submitted: 2015-12-17 07:04

We investigate the properties of a `solar tornado' observed on 15 July 2014, and aim to link the behaviour of the plasma to the internal magnetic field structure of the associated prominence. We made multi-wavelength observations with high spatial resolution and high cadence using the SDO/AIA, the IRIS spectrograph and the Hinode/SOT instrument. Along with spectropolarimetry provided by the THEMIS telescope we have coverage of both optically thick emission lines and magnetic field information. AIA reveals that the two legs of the prominence are strongly absorbing structures which look like they are rotating, or oscillating in the plane of the sky. The two prominence legs, which are both very bright in Ca II (SOT), are not visible in the IRIS Mg II slit-jaw images. This is explained by the large optical thickness of the structures in Mg II which leads to reversed profiles, and hence to lower integrated intensities at these locations than in the surroundings. Using lines formed at temperatures lower than 1 MK, we measure relatively low Doppler shifts on the order of ? 10 km s-1 in the tornado-like structure. Between the two legs we see loops in Mg II, with material flowing from one leg to the other, as well as counterstreaming. It is difficult to interpret our data as showing two rotating, vertical structures which are unrelated to the loops. This kind of `tornado' scenario does not fit with our observations. The magnetic field in the two legs of the prominence is found to be preferentially horizontal.

Authors: Peter Levens, Brigitte Schmieder, Nicolas Labrosse, Arturo Lopez Ariste

Publication Status: ApJ accepted, article in pre-print
Last Modified: 2015-12-19 16:44
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A solar tornado observed by EIS: Plasma diagnostics  

Peter James Levens   Submitted: 2015-08-07 05:36

Context. The term `solar tornadoes' has been used to describe apparently rotating magnetic structures above the solar limb, as seen in high resolution images and movies from the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO). These often form part of the larger magnetic structure of a prominence, however the links between them remain unclear. Here we present plasma diagnostics on a tornado-like structure and its surroundings, seen above the limb by the Extreme-ultraviolet Imaging Spectrometer (EIS) aboard the Hinode satellite. Aims. We aim to extend our view of the velocity patterns seen in tornado-like structures with EIS to a wider range of temperatures and to use density diagnostics, non-thermal line widths, and differential emission measures to provide insight into the physical characteristics of the plasma. Methods. Using Gaussian fitting to fit and de-blend the spectral lines seen by EIS, we calculated line-of-sight velocities and non-thermal line widths. Along with information from the CHIANTI database, we used line intensity ratios to calculate electron densities at each pixel. Using a regularised inversion code we also calculated the differential emission measure (DEM) at different locations in the prominence. Results. The split Doppler-shift pattern is found to be visible down to a temperature of around log T = 6.0. At temperatures lower than this, the pattern is unclear in this data set. We obtain an electron density of log n e = 8.5 when looking towards the centre of the tornado structure at a plasma temperature of log T = 6.2, as compared to the surroundings of the tornado structure where we find log n e to be nearer 9. Non-thermal line widths show broader profiles at the tornado location when compared to the surrounding corona. We discuss the differential emission measure in both the tornado and the prominence body, which suggests that there is more contribution in the tornado at temperatures below log T = 6.0 than in the prominence.

Authors: Peter Levens, Nicolas Labrosse, Lyndsay Fletcher, Brigitte Schmieder
Projects: None

Publication Status: A+A, accepted, article in preprint.
Last Modified: 2015-08-10 11:13
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Structure of prominence legs: Plasma and magnetic field
A solar tornado observed by EIS: Plasma diagnostics

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